1. Field of the Invention
The present invention relates to welded steel/aluminum structures, having characteristics both of aluminum and steel, i.e. lightness and corrosion-resistance of aluminum as well as high strength of steel.
2. Description of Related Art
Aluminum material such as aluminum metal and aluminum alloys has been used in various fields due to lightness and corrosion-resistance. When such aluminum material is used as a member to be subjected to mechanical stress, the aluminum member is necessarily thickened to satisfy a demand for strength. However, the thickening is incompatible with the merit (lightness) of the aluminum member and also with a compact design of an objective structure.
The demand for strength can be attained by lining the aluminum member with a steel part good of mechanical strength without necessity of thickening. Mechanical bonding method, e.g. bolting, riveting or claw clamping, has been employed so far for the lining, although it is improper for formation of a strong joint and inferior of productivity. If an aluminum part is integrated with a steel part by welding, steel/aluminum structures excellent in various properties are provided with productivity remarkably higher than the mechanical bonding method. However, when the aluminum part is integrated with the steel part by a conventional melt welding process, brittle intermetallic compounds are massively formed at a joint boundary, resulting in a significant decrease of joint strength.
Such intermetallic compounds are diffusion reaction products of atoms between steel and aluminum parts. Generation of the intermetallic compounds is suppressed by proper control of welding conditions such as a reaction temperature and a welding time, which affect the atomic diffusion, during friction-welding, as noted in JP 2003-33885A. However, the friction-welding puts significant restrictions on joint designs and rather complicated as compared with other welding processes, resulting in poor productivity. A spot-welding process is also employed for fabrication of steel/aluminum structures. For instance, a hot-dip Al-coated steel sheet is resistance-welded with an aluminum part, as noted in JP 6-39588A.
It is generally thought that a hot-dip Al-coated steel sheet shows similar behaviors to an aluminum part during welding, due to presence of an Al coating layer on its surface. But, a surface part to be welded is heated at a high temperature above a melting temperature (660° C.) of Al during spot-welding. The high-temperature heating causes formation of a molten Al, to which Fe and Si are diffused from an Al—Fe—Si ternary alloy layer at an interface between the steel substrate and the coating layer. Fe is re-precipitated at a cooling step of welding, while Si is distributed to a whole of the molten Al due to its high diffusivity. Consequently, a weld joint has a nugget, wherein a brittle Al—Fe binary alloy layer is formed at a whole area of a joint boundary, resulting in a significant decrease of weld strength.
Unfavorable effects of the Al—Fe binary alloy layer on weld strength are suppressed by controlling a share of a joint boundary occupied by intermetallic compounds. According to a process proposed by JP 2003-145278A, generation of a heat is preferentially performed at a hot-dip Al-coated steel during spot-welding, by holding the steel part and an aluminum part at positive and negative sides, respectively, in order to suppress formation of the intermetallic compounds. However, massive formation of the intermetallic compounds is still unavoidable.